The geometry menu

This menu is for applying geometrical operations to the image. Such operations take no account of the content of each pixel.

Crop is for specifying a rectangular area of the image so that everything outside the rectangle is discarded. Setting the rectangle is done by pointing with the mouse to two opposite corners. Each point can be dragged while the mouse button is down, to set it accurately.

Auto-crop (remove black rim) may be useful after projection or several rotations, when black areas are created outside the image in order to avoid cropping the result of the operation. Auto-crop finds the minimum horizontally aligned rectangle to enclose all non-black pixels (those for which at least one channel has a level greater than 0) and then removes everything outside that rectangle.

Remove RAW margin is only available as an option in this menu if the current image has been loaded as RAW. If you have also set the RAW margins in GRIP's configuration menu for your particular camera, the image will be cropped to remove those margins.

Flip horizontally produces a mirror image by swapping pixels about a vertical mid-axis.

Flip vertically produces a mirror image by swapping pixels about a horizontal mid-axis. Doing this after a horizontal flip is a way of rotating the image exactly by 180 degrees, without any pixel interpolation.

Scale asks for a scale factor and then enlarges or reduces the image uniformly by that factor. Linear interpolation between pixels smooths the resulting image in such cases, so this is not intended for making enlargments with optimum sharpness.

Fit image in m x n first asks for a width and height. The image is then scaled to fit within those dimensions. The width/height ratio of the original image is preserved (pixels remain square) so one of either the resulting width or height may be less than the selected limits. NB: This operation can make the image larger as well as smaller, if the selected width and height are both larger than that of the image. Linear interpolation between pixels smooths the resulting image in such cases, so this is not intended for making enlargments with optimum sharpness. It is meant for scaling down. One use for this is making an image suitable for display on a PC screen so the display software will not need to do the scaling.

Rotate asks for an angle in degrees (anticlockwise) and rotates the image by that amount. Pixel interpolation is done. Generally the size of the resulting image is greater because it is not cropped, so there will be black areas in the corners.

Translate asks for x and y displacement values (whole numbers of pixels) and then shifts the image accordingly. Image size is unchanged. Margins exposed by the shift are filled with zeroes. Portions of the image shifted off the edge are lost. Shifting an image by a very small amount (1 or 2 pixels) and then subtracting it from itself (eg, as previously cloned from the image menu) is a way of detecting edges in the direction of the offset. It is then possible to make line drawings from photos by going on to threshold the edge image, then zeroing the image itself (eg, by using the curves option in the levels menu), drawing the overlay into the image, converting to monochrome, inverting and auto-stretching the result. Here is an example:

Inverse gnomonic (plane to sphere) }

Gnomonic projection (sphere to plane) } these two operations are explained on a separate page. They are necessary if overlapping photos are to be combined. That might be for making a panorama or as part of our multi-exposure astrophotography process.

Help about this menu displays the present page if you have installed files as described here.